Digitally signed (and encrypted) emails can be processed in exactly the same way as regular ones.
Be default Mail.dll extracts signed (encrypted) email content, so you should simply use IMail.Attachments collection and regular MimeData.Save method to save on disk.
Please note, that in case of an encrypted email, you need to provide (or have installed in 'My' certificate store) your private key certificate. This certificate is needed to decrypt email content.
If you have any problems with specific email, please contact us directly. You can find support email address in this page's footer. Please attach raw eml content of the message (possibly zipped).